Continuing pressure for improving the fuel consumption of automobiles and light trucks has raised interest in extracting energy from the heat contained in the exhaust gas that is normally lost when the exhaust gas is vented to the atmosphere. One approach under consideration is to use thermoelectric elements which generate an electrical potential difference when a temperature gradient is established and maintained so that one end of the thermoelectric element is maintained at a higher temperature than the other end. For an individual element, both the potential difference and the current conveyed from the element under such a temperature gradient may be small, but a thermoelectric device fabricated from a plurality of such elements is capable of generating sufficient power to supplement other on-vehicle electrical resources.
A wide range of materials exhibit the thermoelectric effect, but, semiconductor compositions such as PbTe and SiGe are gaining in prominence. For high temperature use, SiGe and Zintl phase compositions such as Yb14MnSb11 may offer some advantage as may skutterudite compositions based on compounds with the general formula MX3 where M may be Co, Ni, or Fe and X may be P, Sb, or As, for example CoP3 and CoAs3. Further examples of semiconductor binary skutterudites, here incorporating platinum group metals, include RhP3, RhAs3, RhSb3, IrP3, IrAs3, and IrSb3.
A suitable elevated temperature skutterudite composition is based on CoSb3, sometimes written as Co4Sb12, which may be doped with one or more dopants of rare-earth, alkaline-earth, or alkali metal elements to render an n-type element. Substitution of Fe or a suitable combination of Fe and Ni for Co, in conjunction with the above-mentioned dopants, results in a p-type element. Alternating p-type and n-type elements may then be arranged into a thermoelectric device. This CoSb3 skutterudite composition is well adapted to extract energy from automotive exhaust gases at temperatures of 400° C. to 600° C.
One end of the elements or the one side of the device may be placed, either in good thermal contact with the exhaust gas stream directly or with the exhaust pipe conveying the exhaust gas to the rear of the vehicle for discharge. The other end of the element or opposing side of the device is thermally coupled to a suitable heat sink to establish the temperature difference across the elements. Commonly the ‘cold’ side of the device may be thermally coupled to atmospheric air or to engine coolant to maintain the cold side at a more or less constant temperature to enable such a temperature difference during all phases of engine operation.
At least a portion of an unsealed thermoelectric device, when used to extract energy from a vehicle exhaust, may be exposed to appreciable temperatures of 500° C. or more in an oxidizing environment. Under these operating conditions the thermoelectric composition may degrade. For example, CoSb3, which does not form a protective oxide, will both progressively oxidize and sublime when exposed to these conditions. Thus, for durability and longevity, the device may be encapsulated or otherwise isolated from exposure to at least air, oxygen, and water vapor when in service at elevated temperatures such as those representative of exhaust gas temperatures. However such encapsulation/isolation should not compromise the intended purpose of the device, that is, to exploit to the temperature difference between the heated exhaust and the environment by generating electricity. Thus the protective structure should neither interpose a significant thermal barrier between the device and the heat source or heat sink, nor provide a shunt, or an alternative pathway for heat flow, which allows an appreciable portion of the heat flow to bypass the device.